1. Field of the Invention
This invention relates to the field of navigation. More specifically, the invention comprises a needle indicator which provides a simple visual reference allowing a user to guide a moving vehicle along a defined course. The invention can use a course defined by waypoints or just a constant track made good (angle over the ground).
2. Description of the Related Art
Aircraft navigation has been focused for the past several decades on radio frequency instruments. One common instrument is a VHF Omni Range station (“VOR”). Those skilled in the art will know that VOR's transmit a signal with two modulations, and that the phase difference between these two modulations is read by airborne instruments to display the radial “from” or bearing “to” the VOR station.
In use, a VOR receiver is typically set to “look” for a specific radial (such as the 180 degree radial, or the 255 degree radial). A visual indicator is provided to inform the pilot when the designated radial is crossed (described in greater detail subsequently).
VOR's are line-of-sight devices. Thus, in flying across the country, a pilot will traditionally navigate from VOR to VOR as they come in range. FIG. 1 shows a cross-country flight using two VOR's for navigation. Upon departing the runway at origin 10, the pilot sets the VOR receiver to “look” for the 180° radial emanating from VOR1. Once this radial is intersected, the pilot flies along the radial toward VOR 1.
Upon reaching VOR1, the pilot resets the VOR receiver to “look” for the 105° radial emanating from VOR2. Once this is intersected the pilot flies along this radial until reaching the proximity of destination 12 (Aircraft 16's actual track along the ground is denoted as ground track 14).
VOR receivers provide a visual indication to the pilot to direct his or her course. FIGS. 2 and 3 show prior art VOR indicators 18 in operation. These are typically a round “gauge” style instrument, having a center 22 and several visual reference markers 20. An indicator needle 24 moves left and right across the surface according to the aircraft's position with respect to the selected radial.
The VOR receiver has a tuning dial, which must be set to the frequency for the desired VOR. A second adjustment allows the user to select the desired radial. In FIG. 2, the 180° radial has been selected. To/From setting 46 reads “To,” indicating that the aircraft is headed toward the VOR station rather than away from it. Various selection switches are customarily provided for such instruments. As these are well known in the art, they have not been illustrated.
In FIG. 2, aircraft 16 is approaching the 180° radial, but the radial still lies off to the aircraft's left. The state of VOR indicator 18 correspond's to the aircraft's position. Indicator needle 24 is off to the left, but is moving toward the center since the aircraft is approaching the selected radial.
In FIG. 3, the aircraft has turned onto a course paralleling the radial (due north), but has overshot slightly. Indicator needle 24 has shifted to right of center—indicating that the radial is off to the right—but will move back toward center as the aircraft's course converges on the radial. A bit of course oscillation is typical as the pilot tries to center and stabilize the needle. This is especially true when first intersecting the radial, as the aircraft's course may be forty degrees or more off the radial's heading.
Returning now to FIG. 1, the reader will appreciate some of the limitations of VOR-based navigation. A direct route between the origin and destination is preferable. Such a route is not available, however, since VOR2 is not within range of the origin airport. As stated previously, VOR's are line-of-sight devices. Their signals can be obstructed by mountains. Their accessibility is also limited by the altitude of the aircraft, with the range being reduced when an aircraft is at low altitude. Navigation instruments using the Global Positioning System (“GPS”) have become widely available in recent years. GPS instruments have the following advantages: (1) They allow point to point navigation, meaning that it is not necessary to pass over unwanted waypoints, as with VOR navigation; and (2) A good signal is generally available at all altitudes, even near the ground in rough terrain.
By the same token, existing GPS navigation devices are known to have certain well-accepted limitations, including: (1) They require the pilot to estimate headings in order to intercept a course line and to then track the course line, increasing the pilot's work load; and (2) They require the pilot to use a generally separate heading device, such as a directional gyroscope (introducing traditional problems such as gyroscopic precession, inaccurate initialization of the gyro compass, etc.).
The increased work load results from the system's reliance on the pilot's mental calculations. The GPS device can provide the pilot with a real-time reading for cross-track error, but the pilot must then mentally estimate an appropriate heading correction to bring the aircraft onto the desired course. An experienced pilot is quite capable of doing this. The pilot will attempt to fly the intercepting course by monitoring the gyro compass or other heading indicator, while simultaneously monitoring the shrinking value of the cross-track error on the GPS cross-track error display.
Performing such a maneuver consumes a substantial portion of the pilot's available working capacity. If he or she is dealing with other factors—such as instrument weather conditions and hazardous terrain—the attention required to perform the intercepting maneuver would be better directed elsewhere.
Some very sophisticated prior art devices solve many of these problems. Commercial airliners often include a “Flight Director.” This instrument uses a computer to blend VOR or other navigational sources with heading information (often supplied from a gyro compass or similar inertial navigation device). A Flight Director gives the pilot immediate visual feedback regarding how much to bank the aircraft to properly intercept and track a selected course. They can also provide visual feedback regarding pitch commands needed to maintain a desired vertical track (such as a planned descent).
Flight Directors have disadvantages, however, including: (1) They are far too expensive for implementation in most small aircraft; (2) They require a modified attitude indicator in order to display the pitch and roll commands; (3) They require an accurate and integrated heading source; and (4) They depend upon the integration of several previously separate systems (attitude indicator, gyro compass etc.). This last fact means that a Flight Director provides little redundancy in the instrumentation. If one of the primary instruments fails, the Flight Director is unlikely to provide meaningful assistance.
Despite the disadvantages explained for the traditional VOR display, the reader will appreciate that the “needle” indicator provides a simple and intuitive means of directing the aircraft. Further, almost all pilots are familiar with using such an instrument. Thus, providing an intuitive indicator such as seen in FIGS. 2 and 3, while eliminating the drawbacks inherent in the traditional navigation instruments is desirable.